While cristobalite has certain advantageous physical properties at high temperatures, it creates serious problems in refractory silica articles because of the extreme volume changes during the crystallographic alpha-beta inversion. The inversion is too often catastrophic in high-density glass products.
The damage caused during the inversion is less severe in a highly porous product so that minor amounts of cristobalite can be tolerated in a ceramic core for metal casting. In modern processes for investment casting of turbine engine parts, such as the well-known directional solidification (D.S.) process, where metal casting temperatures exceed 1500.degree. C., special cores are required which have adequate high-temperature sag-resistance to maintain their shape. One proposed solution to the problem is disclosed in U.S. Pat. No. 4,093,017 wherein a porous ceramic core formed of vitreous silica is provided with small amounts of a metal-oxide mineralizer or crystallization aid to promote the growth of cristobalite.
Unfortunately the process disclosed in that patent cannot be used in silica glass cores sintered to a density of 90 to 95 percent or more because of the catastrophic alpha-beta inversion which occurs when crystalline silica is heated or cooled through the temperature range of 150.degree. to 300.degree. C.
Prior to the present invention, cristobalite has always been considered intolerable in quartz glass crucibles used for growing silicon monocrystal by the Czochralski (Cz) process. The semi-conductor industry has insisted that slip-cast glass crucibles used in Cz crystal-growing furnaces be sintered at a temperature above 1750.degree. C. long enough to eliminate all of the crystalline silica. The industry could not cope with the problems associated with cristobalite and did not know how to obtain commercial benefits from this troublesome crystalline material.
Copending patent application Ser. No. 523,982 filed May 16, 1990 (U.S. Pat. No. 5,053,359) discloses a unique process for making cristobalite crucibles for use in the Cz process using aluminum hydroxide as a crystallization aid. Prior to the present invention, that process has had limited value and has been somewhat unreliable because of problems encountered when using conventional slip-casting methods.
The entire disclosure of said application Ser. No. 523,982 including the drawings is incorporated herein by reference and made a part of the present application to facilitate an understanding of the nature and background of the present invention.
The commercial advantages of slip-casting, which may be suggested by that application, are not achieved in a satisfactory manner in the manufacture of Cz crucibles when using fine particles of crystallization aids, such as aluminum oxide or aluminum hydroxide, which tend to agglomerate in a slurry or slip or which cannot be homogeneously dispersed in the slurry.
Unfortunately slip casting with a slurry having a pH of 10 or higher is not feasible. At the preferred pH of 7 to 8, fine particles of aluminum oxide or aluminum hydroxide will agglomerate and cannot be dispersed as discrete particles to provide a stable homogeneous suspension of the type preferred for slip casting. The agglomeration problem is reduced by raising the pH to 12 or higher, but that would not be acceptable in a slurry used for slip casting or electrophoretic deposition.